The present invention relates to solid state scintillator bars and, more particularly, to coatings for solid state scintillator bars for improving the light output therefrom.
Although the present invention may have more general applications, for concreteness, the invention is described in the environment of an X-ray computer tomography (CT) device.
A CT device employs a fan beam of penetrating radiation passing through a body to be studied and falling on an array of scintillation detectors. Each scintillation detector includes a material capable of emitting light when impinged on by the penetrating radiation and a light detector effective to produce an electrical signal in relation to the light it receives. The electrical signals from the array of scintillation detectors are processed in a computer to construct an image of the portion of the body being studied.
The sensitivity of scintillation detectors is particularly important when the body under study is a part of a living human since increased detector sensitivity permits completion of a given study with reduced radiation exposure.
Early scintillation detectors employing a gas with photomultipliers have given way to solid state scintillators using solid state light detectors. Modern solid state scintillation detectors such as disclosed, for example, in U.S. Pat. Nos. 4,491,732 and 4,525,628, the disclosures of which are herein incorporated by reference as background material, employ a parallelepiped bar of a suitable single-crystal or poly-crystalline scintillating material optically coupled to a photo detector such as, for example, a photo-diode, PIN diode, or photoresistor. Several suitable materials for scintillation bars and light detectors are recited in the referenced patents, and the particular ones employed have no limiting effect on the present invention.
The scintillator bars in a detector array are spaced closely together with one face, called the front face, receiving the radiation. Light emitted within a scintillator bar is transmitted for detection from the face, called the rear face, opposite the front face. Emission of light may take place anywhere within the irradiated volume and may pass in any direction. There is a consequent high probability that emitted light travels toward a face other than the rear face.
A scintillator bar is generally transparent. One technique for encouraging the emitted light to reach the rear face shown, for example in U.S. Pat. Nos. 4,535,243; 3,857,036 and 4,110,621, includes placing specular reflectors on some faces of the scintillator bar. Thus, light impinging on any of the mirrored faces is reflected back into the bar. Eventually, some of the reflected light reaches the rear face and be detected. Several types of loss occur in this process making the light output less than is desired. First, light striking a specularly reflecting surface at close to normal incidence may require many reflections before it reaches the rear face. A good reflector such as, for example, silver, reflects only about 95 percent of the incident light and absorbs the rest. Thus, about five percent of the light is lost on each reflection. After many reflections from the surfaces of the bar, very little of the light is left. Furthermore, practical scintillator materials are not ideally transparent. As a consequence, light is absorbed in each transit through the bar between reflections from the surfaces.
One technique for reducing the number of reflections within a scintillator bar replaces the specularly reflecting surface with a diffuse reflector such as, for example, a paint layer of titanium dioxide. As is well known, titanium dioxide acts as a scatterer of light rather than a reflector. Light impinging on it at normal incidence, for example, is emitted with a generally cosine distribution. Thus, a much greater proportion of the impinging energy is directed back into the bar at shallow angles effective for reaching the rear face with less interaction with the other faces.
Diffuse reflectors rely on the scattering of light by particles of high refractive index rather than on reflection. Thus a relatively thick coating is required to return most of the light to the scinitillator bar. In addition, modern scintillator arrays arrange their scintillator bars in tight proximity without separators therebetween. Optical crosstalk between the scintillator bars is suppressed by making the surface coating substantially opaque. This also requires a relatively thick paint coating on the order of about 0.0014 inch. The opacity may be improved with a coating of an opaque material such as a metal.
Thickness uniformity of the paint coating is critical to performance of the array. To obtain it, the paint coating is hand finished using an abrasive, before applying the opaque coating. Such hand finishing is time consuming and risks damaging the product, thereby increasing the scrap rate.